Using Window Properties

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This section explains how to perform the following tasks associated with window properties.

Adding a Window Property

The following example loads an icon and then a cursor and allocates memory for a buffer. The example then uses the SetProp function to assign the resulting icon, cursor, and memory handles as window properties for the window identified by the application-defined hwndSubclass variable. The properties are identified by the strings PROP_ICON, PROP_CURSOR, and PROP_BUFFER.

#define BUFFER 4096 
 
HINSTANCE hinst;       // handle of current instance 
HWND hwndSubclass;     // handle of a subclassed window 
HANDLE hIcon, hCursor; 
HGLOBAL hMem; 
char *lpMem; 
TCHAR tchPath[] = "c:\\winnt\\samples\\winprop.c";
HRESULT hResult; 
 
// Load resources. 
 
hIcon = LoadIcon(hinst, MAKEINTRESOURCE(400)); 
hCursor = LoadCursor(hinst, MAKEINTRESOURCE(220)); 
 
// Allocate and fill a memory buffer. 
 
hMem = GlobalAlloc(GPTR, BUFFER); 
lpMem = GlobalLock(hMem);
if (lpMem == NULL)
{
// TODO: write error handler
}
hResult = StringCchCopy(lpMem, STRSAFE_MAX_CCH, tchPath);
if (FAILED(hResult))
{
// TO DO: write error handler if function fails.
} 
GlobalUnlock(hMem); 
 
// Set the window properties for hwndSubclass. 
 
SetProp(hwndSubclass, "PROP_ICON", hIcon); 
SetProp(hwndSubclass, "PROP_CURSOR", hCursor); 
SetProp(hwndSubclass, "PROP_BUFFER", hMem);

Retrieving a Window Property

A window can create handles to its window property data and use the data for any purpose. The following example uses GetProp to obtain handles to the window properties identified by PROP_ICON, PROP_CURSOR, and PROP_BUFFER. The example then displays the contents of the newly obtained memory buffer, cursor, and icon in the window's client area.

#define PATHLENGTH 256 
 
HWND hwndSubclass;     // handle of a subclassed window 
HANDLE hIconProp, hCursProp; 
HGLOBAL hMemProp; 
char *lpFilename; 
TCHAR tchBuffer[PATHLENGTH]; 
size_t * nSize; 
HDC hdc;
HRESULT hResult; 
 
// Get the window properties, then use the data. 
 
hIconProp = (HICON) GetProp(hwndSubclass, "PROP_ICON"); 
TextOut(hdc, 10, 40, "PROP_ICON", 9); 
DrawIcon(hdc, 90, 40, hIconProp); 
 
hCursProp = (HCURSOR) GetProp(hwndSubclass, "PROP_CURSOR"); 
TextOut(hdc, 10, 85, "PROP_CURSOR", 9); 
DrawIcon(hdc, 110, 85, hCursProp); 
 
hMemProp = (HGLOBAL) GetProp(hwndSubclass, "PROP_BUFFER"); 
lpFilename = GlobalLock(hMemProp);
hResult = StringCchPrintf(tchBuffer, PATHLENGTH, 
    "Path to file:  %s", lpFilename);
if (FAILED(hResult))
{
// TODO: write error handler if function fails.
}
hResult = StringCchLength(tchBuffer, PATHLENGTH, nSize)
if (FAILED(hResult))
{
// TODO: write error handler if function fails.
}
TextOut(hdc, 10, 10, tchBuffer, *nSize);

Listing Window Properties for a Given Window

In the following example, the EnumPropsEx function lists the string identifiers of the window properties for the window identified by the application-defined hwndSubclass variable. This function relies on the application-defined callback function WinPropProc to display the strings in the window's client area.

EnumPropsEx(hwndSubclass, WinPropProc, NULL); 
 
// WinPropProc is an application-defined callback function 
// that lists a window property. 
 
BOOL CALLBACK WinPropProc( 
    HWND hwndSubclass,  // handle of window with property 
    LPCSTR lpszString,  // property string or atom 
    HANDLE hData)       // data handle 
{ 
    static int nProp = 1;    // property counter 
    TCHAR tchBuffer[BUFFER]; // expanded-string buffer 
    size_t * nSize;          // size of string in buffer 
    HDC hdc;                 // device-context handle
    HRESULT hResult; 
 
    hdc = GetDC(hwndSubclass); 
 
    // Display window property string in client area.
    hResult = StringCchPrintf(tchBuffer, BUFFER, "WinProp %d:  %s", nProp++, lpszString);
    if (FAILED(hResult))
    {
    // TO DO: write error handler if function fails.
    }
    hResult = StringCchLength(tchBuffer, BUFFER, nSize);
    if (FAILED(hResult))
    {
    // TO DO: write error handler if function fails.
    } 
    TextOut(hdc, 10, nProp * 20, tchBuffer, *nSize); 
 
    ReleaseDC(hwndSubclass, hdc); 
 
    return TRUE; 
}

Deleting a Window Property

When a window is destroyed, it must destroy any window properties it set. The following example uses the EnumPropsEx function and the application-defined callback function DelPropProc to destroy the properties associated with the window identified by the application-defined hwndSubclass variable. The callback function, which uses the RemoveProp function, is also shown.

case WM_DESTROY: 
 
    EnumPropsEx(hwndSubclass, DelPropProc, NULL); 
 
    PostQuitMessage(0); 
    break; 
 
// DelPropProc is an application-defined callback function 
// that deletes a window property. 
 
BOOL CALLBACK DelPropProc( 
    HWND hwndSubclass,  // handle of window with property 
    LPCSTR lpszString,  // property string or atom 
    HANDLE hData)       // data handle 
{ 
    hData = RemoveProp(hwndSubclass, lpszString); 
//
// if appropriate, free the handle hData
//
 
    return TRUE; 
}